Digital-to-analog converter

Abstract

A digital-to-analog converter comprises a sigma-delta modulator for generating a 1-bit digital signal (D3) modulated synchronously with a clock signal (NF S ) in response to a digital input signal (D2). This modulator is followed by an asynchronous analog sigma-delta modulator for generating a bivalent asynchronously modulated signal (D4) in response to the synchronously modulated digital signal (D3). The information in the digital signal (D3) from the sigma-delta modulator resides in signal transitions which can appear exclusively at discrete instants defined by the clock signal. The information in the asynchronously modulated signal (D4) from the asynchronous sigma-delta modulator is contained in an analog variation of the duty cycle. This means that the signal transitions of the asynchronously modulated signal (D4) are not tied to a fixed pattern of discrete instants but that all the intervening instants are also available. As a result of the freedom of the positions of the signal transitions, the frequency spectrum of the asynchronously modulated signal (D4) has a substantially smaller width than the frequency spectrum of the synchronously modulated digital signal (D3) from the sigma-delta modulator. This enables an interface to a further signal processing device to be simplified.

I claim: 
     
       1. A digital-to-analog converter comprising: a sigma-delta modulator for generating a 1-bit digital signal (D3) modulated synchronously with a clock signal (NF S ) in response to a digital input signal (D2), output means for supplying an output signal in response to the synchronously modulated digital signal (D3), and wherein the digital-to-analog converter further comprises an asynchronous sigma-delta modulator for generating a bivalent asynchronously modulated signal (D4) in response to the synchronously modulated digital signal (D3) from the sigma-delta modulator and for supplying the asynchronously modulated signal (D4) to the output means. 
     
     
       2. A digital-to-analog converter comprising: a sigma-delta modulator having input means for receiving a digital input signal and operative to generate at its output a 1-bit digital signal modulated synchronously with a clock signal in response to said digital input signal,   an asynchronous sigma-delta modulator having input means coupled to the output of said sigma-delta modulator to receive said 1-bit synchronously modulated digital signal and being operative in response to the 1-bit synchronously modulated digital signal to generate a bivalent asynchronously modulated signal at its output,   an analog output filter means, and   an interface circuit coupled between the output of the asynchronous sigma-delta modulator and an input of the analog output filter means.   
     
     
       3. The digital-to-analog converter as claimed in claim 2 further comprising: an input terminal for an M-bit digital signal (D1) having a clock frequency F s , and   an upsampling filter coupled between said input terminal and said input means of the sigma-delta modulator and having a sampling input for a clock frequency signal NF s  whereby the upsampling filter produces at its output an M-bit digital signal (D2) having a clock frequency NF s , where N is a number greater than 1.   
     
     
       4. The digital-to-analog converter as claimed in claim 2, wherein said interface circuit comprises an inverter and said analog output filter means comprises a low pass filter. 
     
     
       5. The digital-to-analog converter as claimed in claim 2 wherein said asynchronous sigma-delta modulator converts an input digital signal (D3) having a wide frequency spectrum into an output digital signal (D4) having a much narrower frequency spectrum. 
     
     
       6. The digital-to-analog converter as claimed in claim 5 wherein the asynchronous sigma-delta modulator output digital signal (D4) does not include any frequencies above 40 MHz when the input digital signal (D3) comprises a clock frequency of 200 MHz modulated with a sine wave of 1 MHz. 
     
     
       7. The digital-to-analog converter as claimed in claim 2 wherein: the sigma-delta modulator comprises a cascade connection of a digital differential circuit coupled to the input means of the sigma-delta modulator, a digital low pass filter, and a digital quantizer having an output coupled to the output of the sigma-delta modulator, and a feedback circuit coupled between the output of the digital quantizer and an input of the digital differential circuit, and   the asynchronous sigma-delta modulator comprises a cascade connection of an analog differential circuit coupled to the input means of the asynchronous sigma-delta modulator, an analog low pass filter, and an analog comparator having an output coupled to the output of the asynchronous sigma-delta modulator, and a feedback circuit coupled between the output of the analog comparator and an input of the analog differential circuit.   
     
     
       8. The digital-to-analog converter as claimed in claim 2 wherein the sigma-delta modulator comprises a first integrated circuit and the analog output filter means comprises a second integrated circuit. 
     
     
       9. The digital-to-analog converter as claimed in claim 2 further comprising a notch filter coupled between the output of the sigma-delta modulator and the input means of the asynchronous sigma-delta modulator. 
     
     
       10. In a digital video system, a digital-to-analog converter comprising: a sigma-delta modulator having input means for receiving a video frequency digital input signal and operative to generate at its output a 1-bit digital signal modulated synchronously with a clock signal at a frequency that is N times the video frequency in response to said digital input signal,   an asychronous sigma-delta modulator having input means coupled to the output of said sigma-delta modulator to receive said 1-bit synchronously modulated digital signal and being operative in response to the 1-bit synchronously modulated digital signal to generate a bivalent asynchronously modulated signal at its output,   an analog output filter means, and   an interface circuit coupled between the output of the asychronous sigma-delta modulator and an input of the analog output filter means.